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I am sorry to learn of this situation. Unfortunately there isn't a defense to this action.
Many people engage in offshore gambling (which is illegal) and use their credit card incurring a debt (which is enforceable), and there is no recourse (you cannot tell your creditor that you shouldn't have to pay because you were spending the money improperly).
See generally: http://www.vjolt.net/vol6/issue1/v6i1a06-Anastasio.html
Depending on your other accounts and assets, bankruptcy may be a consideration for you, but if you do consider this route, please speak with a local bankruptcy attorney (while it is possible to file for bankruptcy protection without an attorney, you will not gain the full benefit of filing as a bankruptcy attorney can help you maximize your post-discharge financial position, and this is especially important when there is a claim (such as in this case) that the debtor was engaged in illegal activity, which can make the debt survive bankruptcy protection!!!).
If you were "duped" into this type of transaction, you might have a case. (So if the circumstances were different, you may have a point, but even then, many businesses bill under different accounts and there is nothing fraudulent about it).
But that really isn't what happened here. What really happened is that you gambled online, and they used a pass through to evade US and State laws on gambling in order to conduct illegal activity.
You really don't have a good argument to get around this.
I am sorry, I wish I wish I had a different answer for you on this one.